1. Field of the Invention
The present invention relates to light emitting semiconductor devices employing nitride semiconductor and methods of fabricating the same.
2. Conventional Art
Using nitride semiconductor material of GaN, InN and AlN and a semiconductor of a mixture thereof, light emitting devices with InxGa1-xN crystal as a light emitting layer on a sapphire substrate, GaN substrate, SiC substrate or silicon (111) substrate have been produced. Since the Si substrate is particularly superior than the other substrates by the advantage of providing those of a large area and constant quality at low cost, it is expected that a light emitting device can be produced at low cost by using an Si substrate. Furthermore, an attempt is also being made to prototype a light emitting semiconductor device using nitride semiconductor material formed of the above mixed crystal semiconductor.
When a nitride semiconductor device is fabricated on the sapphire substrate, SiC substrate, silicon (111) substrate as described above, a nitride semiconductor film is obtained typically with a hexagonal crystal's C plane as a growth surface.
With the C plane as a growth surface, however, polarization from anisotropy is readily applied to the c axis direction as a piezoelectric field as a hexagonal crystal is a uniaxial crystal. As such, when an active layer is stacked in the c axis direction and its plane is used, from the piezoelectric field electron and hole carriers are separated to opposite ends of a triangular potential, as shown in FIG. 1A, and accordingly the electron and hole carriers are hardly recombined.
Although in the active layer a well layer is increased in thickness to prevent emission from providing longer wavelength and reduce quantum effect, the piezoelectric field's effect that further separates electron and hole carriers (FIG. 1B) provides a reduced carrier recombination probability. As such, when a long-wavelength light emitting nitride semiconductor device is fabricated, changing the composition of a well layer of mixed crystal semiconductor in an active layer to control a bandgap is the only approach.
For example in an experiment conducted by the present inventors when a nitride based semiconductor material with a C plane as a main plane was used to fabricate a light emitting semiconductor device on a sapphire substrate the light emitting device with an active layer having a thickness of 3 nm provided maximum luminance and the luminance is approximately halved for a thickness ranging from 2 to 4.5 nm.
As such, when a nitride based, light emitting semiconductor device is used to fabricate a light emitting device allowing multicolor emission with a single chip, a light emitting layer different in bandgap is used to form a multilayer structure or an integration, as disclosed in Japanese Patent Laying-Open Nos. 7-183576, 8-88407 and 11-87773. In other words, to fabricate a light emitting diode allowing multicolor emission with a single chip, changing a composition of a well layer of mixed crystal semiconductor to provide an appropriately adjusted bandgap is the only approach.